Grain dryers have, for the most part, been fairly wasteful of heat, and the heated air is usually passed through the grain column only once. In some instances, such as in U.S. Pat. No. 4,020,561, the grain column is treated by air of progressively lower temperatures from a number of separate heaters in the drying column and then cooler air is drawn through the grain column for cooling the grain prior to discharge. The U.S. Pat. No. 3,526,969 shows a similar general arrangement wherein the column of grain is initially subjected to high temperatures for drying and then lower temperatures for cooling the heated grain.
There have been some instances of recirculating air in such structures as disclosed by U.S. Pat. No. 3,237,315, but in this structure, instead of using the hottest air to produce the initial and greatest degree of drying, the hot air which has been circulated twice through the grain column and cooled is again circulated through the wettest portion of the grain column with minimal effect. Further, this patent unnecessarily stresses the grain by suddenly applying the coldest air in the cooling process to the grain which has just prior been heated to the maximum extent by the hottest air in the grain dryer. Other grain dryers such as those described in U.S. Pat. Nos. 3,727,323 and 4,006,536, and 3,373,503, and 2,437,899, and 2,740, 204 show various air circulating patterns, none of which is very efficient from the standpoint of conservation of heat or fuel used to operate the burners in the heating units.